Vibrating flow devices such as, for example, densitometers and Coriolis flow meters are used for measuring a characteristic of flowing materials, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating flow devices include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. At least one driver vibrates the one or more conduits at a resonance frequency in one or more of these drive modes for purposes of determining a characteristic of the flowing material. One or more meter electronics transmit a sinusoidal drive signal to the at least one driver, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The drive signal causes the driver to vibrate the one or more conduits at the drive frequency in the drive mode. For example, the drive signal may be a periodic electrical current transmitted to the coil.
At least one pick-off detects the motion of the conduit(s) and generates a sinusoidal pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. However, it should be appreciated that other pick-off arrangements exist such as for example, optical, capacitance, piezo-electric, etc. The pick-off signal is transmitted to the one or more electronics; and according to well known principals the pick-off signal may be used by the one or more meter electronics to determine a characteristic of the flowing material or adjust the drive signal, if necessary.
Typically, vibrating flow devices are provided with two vibrating conduits that vibrate in opposition to each other in order to create an inherently balanced system. As a result, the vibrations from each conduit balance each other out in a manner that prevents undesired vibrations from one conduit from passing to the other conduit. There are, however, certain applications where dual conduits are undesirable, for example, due to problems with pressure drops or clogging. In such situations a single conduit system may be desirable.
Imbalance in single conduit systems arises due to the fact that pick-offs measure motion by determining relative position between a first pick-off component located on a reference member and a second pick-off component located on the conduit. Accordingly, undesirable vibrations that pass to the reference member may cause the component of the pick-offs located on the reference member to vibrate or move in an undesirable manner. This, in turn, may affect the sensed relative position of the pick-off components and generate inaccurate pick-off signals. Furthermore, in some systems, the reference member is designed to vibrate in opposition to the flow conduit. However, if the density of the fluid flowing through the conduit changes, the reference member may not be able to counter the vibrations of the flow conduit.
Attempts at solving this problem have involved using a dummy tube mounting structure that is attached to the conduit via brace bars, and using the motion of the dummy tube to balance the system. While this approach has been somewhat adequate in certain situations, it is generally difficult to balance the system over a wide fluid density range thereby limiting the effectiveness of the prior art approach.
The present invention overcomes these and other problems and an advance in the art is achieved.